The present invention generally relates to quadrupole mass spectrometers. In particular, the present invention relates to a miniature micromachined ion filter for use in a quadrupole mass spectrometer, a quadrupole mass spectrometer including the ion filter, and methods of making the ion filter and the quadrupole mass spectrometer.
Mass spectrometers are workhorse instruments finding applications in many commercial and military markets, with potential for use in domestic markets as well. A mass spectrometer is able to sample, in situ, the atmosphere in which it is placed and provide a reading of the atomic and molecular species (and any positive or negative ions) present in that atmosphere and of the absolute abundance of these species.
There are many types of mass spectrometers, such as magnetic sector, Paul or Penning ion trap, trochoidal monochromator, and the like. One popular type of mass spectrometer is the quadrupole mass spectrometer (QMS), first proposed by W. Paul (1958). In general, the QMS separates ions with different masses by applying a direct current voltage and a radio frequency (xe2x80x9cRFxe2x80x9d) voltage on four rods having hyperbolic or circular cross sections and an axis equidistant from each rod. Opposite rods have identical potentials. The electric potential in the quadrupole is a quadratic function of the coordinates.
Ions are introduced in a longitudinal direction through a circular entrance aperture located at the ends of the rods and centered on the midpoints between rods. Ions are deflected by the field depending on their atomic mass-to-charge (m/z) ratio. By selecting the applied voltage amplitude and frequency of the RF signal, only ions of a selected m/z ratio exit the QMS along the axis of a quadrupole at the opposite end and are detected. Ions having other m/z ratios either impact the rods and are neutralized or deflect away from the centerline axis of the quadrupoles.
As explained in Boumsellek, et al. (1993), a solution of Mathieu""s differential equations of motion in the case of round rods provides that to select ions with a m/z ratio using an RF signal of frequency f and rods separated by a contained circle of radius distance R0 the peak RF voltage V0 and DC voltage U0 should be as follows:
V0=7.233 mf2R20
U0=1.213 mf2R20
Conventional QMS""s weigh several kilograms, have volumes of the order of 104 cm3, and require 50-100 watts of power. Further, these devices usually operate at vacua in the range of 10xe2x88x926-10xe2x88x928 torr in order that the mean free path be comparable to the instrument dimensions, and where secondary ion-molecule collisions cannot occur. Commercial QMS""s of this design have been used for characterizing trace components in the atmosphere (environmental monitoring), automobile exhausts, chemical-vapor deposition, plasma processing, and explosives/controlled-substances detection (forensic applications). However, such conventional QMS""s are not suitable for spacecraft life-support systems and certain national defense missions where they have the disadvantages of relatively large mass, volume, and power requirements. A small, low-power QMS would find a myriad of applications in factory air-quality monitoring, pollution detection in homes and cars, protection of military sites, and protection of public buildings and transportation systems (e.g., airports, subways, and harbors) against terrorist activities.
One type of miniature QMS (U.S. Pat. No. 5,401,962) was developed by Ferran Scientific, Inc., San Diego, Calif. and includes a miniature array of sixteen rods comprising nine individual quadrupoles. The rods are supported only at the detector end of the QMS by means of powdered glass that is heated and cooled to form a solid support structure. The electric potential and RF voltage are applied by the use of springs contacting the rods. The Ferran QMS dimensions are approximately 2 cm diameter by 5 cm long, including a gas ionizer and detector, and has an estimated mass of 50 grams. The reduced size of the Ferran QMS results in several advantages over existing QMS""s, including a reduced power consumption and a higher operating pressure.
The Ferran QMS has a resolution of approximately 1.5 amu in the mass range 1-95 amu. This is a relatively low resolution for a QMS, making the miniature Ferran QMS useful for commercial processing (e.g., chemical-vapor deposition, blood-plasma monitoring) but not for applications that require accurate mass separation, such as in analytical chemistry and in spacecraft life-support systems. Boumsellek et al. (1993) traced the low resolution to the fact that the rods were aligned only to within a xc2x13% accuracy, whereas an alignment accuracy in the range of xc2x10.1% is necessary for a high resolution QMS.
A separate miniature QMS (U.S. Pat. Nos. 5,596,193 and 5,719,393) was developed by the Jet Propulsion Laboratory (JPL), California Institute of Technology to address the continuing need for a reduced size QMS having an acceptable rod alignment. The JPL QMS provides improved resolution over the Ferran QMS due to improved accuracy in rod alignment. As may be appreciated, the accurate positioning and alignment of individual miniature rods in an array significantly increases the cost of manufacturing due to the increased time and specialized equipment required for precisely aligning separate miniature rods. As the size of the rods is further reduced, the complexity, difficulty and expense of rod positioning and alignment increases. In this regard, there is a need for a small QMS having high resolution that may be made by simpler and less expensive manufacturing process.
In one aspect, the present invention provides a quadrupole ion filter, and a quadrupole mass spectrometer including the ion filter, that avoids problems associated with miniaturization of conventional quadrupole mass spectrometer devices, and especially problems concerning the incorporation of loose rods into conventional devices. The ion filter includes a patterned layer of electrically conductive material, with the patterned layer including a two-dimensional array of poles for one or more quadrupoles. Alternatively, the ion filter may be described as a pole array. The pole array, or array of poles, in the pattern is two-dimensional in that the poles in the array have a regular spacing in the x-y plane, with the length of the poles in the array being in the z direction. The poles of the ion filter serve the same function as the rods in conventional quadrupole devices. The patterned layer is divided into a number of separate sections, or pieces, each including at one terminal end one pole in the array of poles. At the other terminal end of each separate piece is a bonding location for convenient electrical connection of the piece with an external power source.
Structurally, the quadrupole ion filter of the present invention is considerably different than the quadrupole structure in conventional quadrupole mass spectrometers. Conventional quadrupole mass spectrometers, even those that have been miniaturized, use poles that are in the form of individual longitudinally extending rods. The ion filter of the present invention, however, includes the array of poles in a thin patterned layer, with the thickness of the layer corresponding with the length of the poles.
The patterned layer in the ion filter of the present invention typically has a thickness of smaller than about 6 millimeters, although even smaller thicknesses may be preferred for some applications. In that regard, the thinner that the patterned layer is, the shorter the length of poles and, therefore, the shorter the distance that ions must travel to pass through the ion filter. A shorter length of travel through the ion filter permits operation at higher pressures, which is a significant advantage with the ion filter of the present invention.
By use of the patterned layer in the ion filter of the present invention, it is possible to make the poles of an extremely small size and with an extremely dense spacing. For example, with the present invention, the density of poles in the patterned layer is typically greater than about 2 poles per square millimeter, and in many embodiments the density is much higher. Furthermore, directly opposing poles in the patterned layer are typically separated by a distance of shorter than about 0.2 millimeter, and in many embodiments by an even shorter distance. Diagonally opposing poles in the patterned layer are typically separated by a distance of shorter than about 0.3 millimeter, and in many embodiments by an even shorter distance. Because of the extremely small size and dense spacing of the poles, the ion filter may include a large array of poles in a small space, with different groupings of four adjacent poles each defining a channel for passage of ions. With the present invention, however, these quadrupole channels are extremely small. When the ion filter includes a large array of poles, defining a plurality of quadrupole channels, the channels are typically present in a density of larger than about one of the quadrupole channels per square millimeter, and often greater than two of the quadrupole channels per square millimeter.
An advantageous structure for the ion filter of the present invention is one in which substantially all of the patterned layer is supported by a single, common supporting substrate, which is typically of dielectric material. The patterned layer is such, however, that a portion of the patterned layer that includes the poles is suspended from the substrate. Typically, the suspended portion of the patterned layer extends over an opening that passes through the substrate. In this way, the opening provides a passageway to permit ions access to the quadrupole channels. The patterned layer is bonded to the supporting substrate in a manner that maintains positioning and alignment of the poles, even though the poles are suspended from the substrate.
A significant aspect of the present invention is manufacture of the quadrupole ion filter, and manufacture of quadrupole mass spectrometers including the ion filter. According to the present invention, a method is provided in which the poles in the patterned layer are made in a manner such that as the poles are made they have relative positioning and alignment for final use in a quadrupole mass spectrometer. This is typically accomplished, according to the method of the present invention, by forming the patterned layer of the ion filter on a common supporting substrate so that the patterned layer, as formed on the common supporting substrate, is bound to the substrate, such that the relative positioning and alignment of poles in the patterned layer is thereby fixed.
One preferred embodiment of the method for manufacturing the ion filter involves simultaneous manufacture of the patterned layer, including the poles, by filling a mold with electrically conductive material. The mold includes a template for the patterned layer. The mold is filled when it is situated on the surface of the common supporting substrate. When the mold is then removed, the patterned layer remains supported by the common supporting substrate. In one embodiment, the mold may be made by a technique known as Lithographie-Galvanoformung-Abformung (LIGA) manufacture.
Another embodiment of the method for manufacturing the present invention involves forming the patterned layer from a single work piece, typically in the form of a metallic sheet, that has been bonded to the common supporting substrate. Material is selectively removed from the work piece to form the patterned layer, such that the patterned layer, as formed, is bound to and supported by the common supporting substrate. Typically, the selective removal of material from the work piece is accomplished by electrical discharge machining (EDM).
The present invention also involves a quadrupole mass spectrometer including the mass filter of the present invention. The quadrupole mass spectrometer includes the ion filter located between an ion source and an ion detector. During operation, the ion source supplies ions to be filtered by the ion filter. Ions passing through the ion filter may then be detected by the ion detector. The quadrupole mass spectrometer may include spacers before and/or after the ion filter to maintain a predetermined spacing between the ion filter and the ion source and/or the ion detector and to assist in isolating the operation of the ion filter from influences from other components. These spacers are typically made of dielectric material. The quadrupole mass spectrometer may also include entrance and/or exit devices for enhancing performance of the quadrupole mass spectrometer. The entrance device is located between the ion source and the ion filter and typically-includes a body of dielectric material having apertures therethrough for channeling ions from the ion source into the ion filter. In a preferred embodiment, the entrance device includes an electrically conductive metallic film at least on a side facing the ion source, to dissipate the charge of ions striking the entrance device. The exit device similarly includes a body of dielectric material having apertures therethrough for channeling ions exiting the mass filter to the ion detector. In a preferred embodiment, the exit device includes an electrically conductive metallic film on at least a side facing the ion filter, to dissipate the charge of ions striking the exit device.
Furthermore, the quadrupole mass spectrometer has a versatile design that may be adapted to a variety of situations. For example, a Faraday-type ion detector may be used for operation at relatively high pressures, often in the millitorr range. For operation of the device at very low pressures, such as those below about 10xe2x88x924 torr, a single particle multiplier may be used as the ion detector.
Also, according to the present invention, the quadrupole mass spectrometer including the ion filter may easily be manufactured through proper alignment and assemblage of the individual components.